Reactive oxygen species affect the potential for mineralization processes in permeable intertidal flats

Intertidal permeable sediments are crucial sites of organic matter remineralization. These sediments likely have a large capacity to produce reactive oxygen species (ROS) because of shifting oxic-anoxic interfaces and intense iron-sulfur cycling. Here, we show that high concentrations of the ROS hydrogen peroxide are present in intertidal sediments using microsensors, and chemiluminescent analysis on extracted porewater. We furthermore investigate the effect of ROS on potential rates of microbial degradation processes in intertidal surface sediments after transient oxygenation, using slurries that transitioned from oxic to anoxic conditions. Enzymatic removal of ROS strongly increases rates of aerobic respiration, sulfate reduction and hydrogen accumulation. We conclude that ROS are formed in sediments, and subsequently moderate microbial mineralization process rates. Although sulfate reduction is completely inhibited in the oxic period, it resumes immediately upon anoxia. This study demonstrates the strong effects of ROS and transient oxygenation on the biogeochemistry of intertidal sediments.

for sediments collected in May and July 2020, and in deep sediment (10-14 cm depth) for sediments collected in July 2020.

Tested concentration range (µM)
Sensitivity (pA µM -1 ) Interference (%) Ammonium 0 -40 9 × 10 -7 1 Nitrite 0 -60 1 × 10 -5 11 Nitrate 0 -30 2 × 10 -6 2 Formate 0 -20 1 × 10 -6 1 Acetic acid 0 -40 5 × 10 -7 0.6 Ascorbate 0 -40 1 × 10 -4 111 Fe 2+ 0 -430 1 × 10 -5 11 Fe 2+ (sensor with ferrozine) none The sensor consisted of an etched 50 µm-thick platinum anode plated with platinum chloride (8% PtCl4 in MilliQ water), an etched 100 µm-thick platinum guard, and a thick platinum reference. The anode, guard and reference were mounted in a glass casing, with the sensing anode at a distance of ca 50 µm from the tip. The tip of the outer capillary had a diameter of 25 -30 µm and a tip opening of 10 µm. Before mounting the electrodes, the tip of the outer capillary was sealed by a thin polyurethane membrane (D6) 1 . The membrane was dissolved in tetrahydrofuran (50 mg mL -1 ) and applied by shortly immersing the capillary in the solution that is kept in the tip of a Pasteur pipette and left to cure overnight. The membrane was applied under microscopic guidance. The membrane separated the electrolyte from the seawater but was permeable for hydrogen peroxide. After mounting the electrodes in the casing, the sensor was filled with electrolyte, a borate/potassium chloride buffer (50 mM borate, 3 M potassium chloride and 500 µM ferrozine), with pH 9.
The selectivity was assessed by addition of potentially interfering compounds (ammonium, nitrate, nitrite, formate, acetic acid, ascorbate, and Fe 2+ (Fe 2+ additions tested at pH 3)). The response of the sensor was assessed for incremental additions of these compounds, and for hydrogen peroxide (3% hydrogen peroxide stock solution). Sensitivity was defined as the slope of the linear trend line. The sensitivity for hydrogen peroxide was 9 × 10 -5 pA µM -1 . Interference (in %) was calculated as: sensitivity interfering compound/sensitivity H2O2 × 100 (Supplementary Table 7; Supplementary Fig. 12).
When no ferrozine was added to the electrolyte, the sensor had strong interference with Fe 2+ ( Supplementary Fig. 12). Sensors with ferrozine in the electrolyte had no response to Fe 2+ ( Supplementary Fig. 2). The sensitivity for Fe 2+ of a sensor with ferrozine (50 µM) was tested by step-wise addition of Fe 2+ to 500 mL of N2-flushed seawater of pH 3. Fe 2+ was tested in final concentrations of 20, 100, and 200 µM, which covers the range of Fe 2+ concentrations measured ( Supplementary Fig. 3), and the signal of the sensor was recorded over time. Ferrozine-filled sensors (with a ferrozine concentration of 500 µM) were used for the measurements of this study.
The sensor was connected to a picoammeter and polarized at +700 mV until reaching a stable current, which happened normally within an hour. The medium in which the sensor was used was connected to an external reference electrode. The sensors were calibrated before use in a stirred beaker with filtered seawater to which aliquots of stabilized 3% hydrogen peroxide were added.
The response times were <3 seconds. The sensor was very stable and not noise sensitive. The sensitivity was 2 × 10 -4 pA µM -1 (Supplementary Fig. 13). The response to hydrogen peroxide was linear within the range of 0.5 µM to 4.3 mM ( Supplementary Fig. 13), the highest concentration tested. The sensor was slightly light-sensitive. The shelf lifetime of the sensor was a few weeks.
With use in anoxic sediments the sensitivity quickly goes down after a while.